1. Field of the Invention
The present invention relates to a voltage controlled oscillator (VCO) and more particularly to a voltage controlled oscillator for use in a phase locked loop system.
A voltage controlled oscillator is an oscillator in which a change in input voltage results in a predetermined change in output frequency. Typically, frequency tuning is accomplished by changing the voltage across a varactor diode in the frequency determining resonant network of the oscillator. Voltage controlled oscillators are very often employed in phase lock loop systems. The phase locked loop system also includes a phase detector and an error amplifier. The phase detector provides an error signal that locks the voltage controlled oscillator to the frequency and phase of an incoming signal.
2. Description of the Prior Art
A typical prior art voltage controlled oscillator is illustrated in FIG. 1. The oscillator is comprised of an LC resonant circuit including capacitors 10, 12 and 14, inductor 16, inverter 18, resistor 20 and varactor diode 22. The varactor is a diode that exhibits a change in capacitance with a change in applied voltage. By changing the applied voltage at an input terminal 24, the capacitance of the varactor will change, thus changing the resonant frequency of the resonant circuit and altering the frequency of an output signal at terminal 26. A bias voltage is applied to the other terminal of the varactor to determine the operating characteristics of the oscillator, i.e., to control the voltage across the varactor so that a predetermined frequency output will be achieved for a predetermined voltage input. The bias voltage is applied by means of a bias circuit including resistors 28 and 30 and variable resistor 32. The circuit is manually adjusted by applying a predetermined voltage input to the terminal 24 and adjusting the variable resistor 32 until the desired output frequency is achieved at the output terminal 26.
As an alternative to the provision of a variable resistor which allows adjustment of the bias voltage, another prior art approach has been to employ extremely close tolerance components in the circuit to ensure that the desired operating characteristics will be achieved. This approach has the advantage that manual adjustment of the circuit is not required. However, the use of close tolerance components results in an expensive circuit.
The operating characteristics of the voltage controlled oscillator may vary over time due to various factors, thus causing the oscillator to have operating characteristics other than those desired. The prior art oscillators described above generally do not compensate for such variations. It is an object of the present invention to provide an oscillator which automatically maintains the desired operating characteristics.